Camera, optical system and interchangeable lens with birefringent device

ABSTRACT

An optical system ( 110 ) includes a lens unit ( 112 ) with a plurality of lenses. An out-of-focus point spread function of the lens unit ( 112 ) includes an annular intensity distribution with at least one ring-shaped side peak at a radial distance to a center point. A birefringent device ( 115 ) in an optical path of the optical system ( 110 ) is adapted to selectively attenuate the ring-shaped side peak in the out-of-focus point spread function of the lens unit ( 112 ).

BACKGROUND

Field of the Disclosure

The present disclosure relates to a camera, an optical system and aninterchangeable lens including a birefringent device, respectively. Thedisclosure further relates to a method of manufacturing an opticalsystem.

Description of Related Art

When taking an image with a lens that limits the depth of focus,out-of-focus portions of the image appear unsharp or blurred. Thesubjective quality of how the out-of-focus portions appear to a viewercontributes to the overall image quality. The way the lens rendersout-of-focus points of light is described by the term “bokeh”. Forexample, out-of-focus light points imaged into spots, in whichbrightness gradually and smoothly decreases with increasing distance toa centre point, result in smoothly blurred out-of-focus portions and apleasant bokeh. Out-of-focus light points which the lens images intoblurred spots in which brightness abruptly decreases or increases at acertain distance to the center point may result in hard edges within theblurred image portions or in rings which the viewer typically perceivesas less pleasant, respectively. Determining factors for the type ofbokeh are amount and sign of the optical aberrations present in theoptical system.

There is a need for optical systems and cameras with pleasant bokeh. Theobject of the present embodiments is achieved by the subject-matter ofthe independent claims. The dependent claims define further embodiments.

SUMMARY

The foregoing paragraphs have been provided by way of generalintroduction and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

According to an embodiment, an optical system includes a lens unit witha plurality of lenses. An out-of-focus point spread function of the lensunit includes an intensity distribution with a ring-shaped side peak ata radial distance to a center point. A birefringent device in an opticalpath of the optical system is adapted to selectively attenuate thering-shaped side peak in the out-of-focus point spread function of thelens unit.

According to another embodiment, an interchangeable lens includes a lensunit with a plurality of lenses. An out-of-focus point spread functionof the lens unit includes an intensity distribution with a ring-shapedside peak at a radial distance to a center point. A birefringent devicein an optical path of the interchangeable lens is adapted to selectivelyattenuate the ring-shaped side peak in the out-of-focus point spreadfunction of the lens.

According to a further embodiment, a camera includes a lens unit with aplurality of lenses. An out-of-focus point spread function of the lensunit includes an intensity distribution with a ring-shaped side peak ata radial distance to a center point. A birefringent device in an opticalpath of the lens unit is adapted to selectively attenuate thering-shaped side peak in the out-of-focus point spread function of thelens unit.

A further embodiment concerns a method of manufacturing an opticalsystem. The method includes determining an out-of-focus point spreadfunction of a lens unit that includes a plurality of lenses. The methodfurther includes determining, by using a numerical optimization method,a polarization dependent phase alteration for the pupil function. Thephase alteration attenuates a ring-shaped side peak in the out-of-focuspoint spread function of the lens unit by at least 25 percent. Thenumerical optimization method varies two complex conjugate phasealterations in combination with the pupil function of the lens unit forminimizing the side peaks at minimum effect on an in-focus point spreadfunction of the lens unit.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following drawings, like reference numerals designate identicalor corresponding parts throughout the several views. The elements of thedrawings are not necessarily to scale relative to each other. Featuresof any of the illustrated embodiments can be combined with one or moreof the other illustrated embodiments to form further embodiments.

FIG. 1A is a schematic block diagram of an optical system with abirefringent device arranged between a first portion of a lens unit andan image capture unit.

FIG. 1B is a schematic plan view of a birefringent device according toan embodiment.

FIG. 1C is a schematic cross-sectional view of the birefringent deviceof FIG. 1B along line C-C.

FIG. 1D is a diagram illustrating out-of-focus point spread functions ofthe optical system of FIG. 1A with and without the birefringent deviceof FIGS. 1B and 1C.

FIG. 1E is a diagram illustrating in-focus point spread functions of theoptical system of FIG. 1A with and without the birefringent device ofFIGS. 1B and 1C.

FIG. 1F is a diagram illustrating modulation transfer functions of theoptical system of FIG. 1A with and without the birefringent device ofFIGS. 1B and 1C.

FIG. 2A is a schematic plan view of a further birefringent deviceaccording to a further embodiment.

FIG. 2B schematically illustrates wavefront coding along two orthogonalaxes of the birefringent device of FIG. 2A for discussing effects of theembodiments.

FIG. 3A is a schematic block diagram of an optical system according toan embodiment with a birefringent device arranged between an entrance ofan optical system and a lens unit.

FIG. 3B is a schematic block diagram of an optical system with atwo-piece birefringent device arranged between a first portion of a lensunit and an image capture unit.

FIG. 3C is a schematic block diagram of an optical system according toan embodiment concerning a controllable birefringent device.

FIG. 4A is a schematic cross-sectional view of a camera with aninterchangeable lens including a birefringent device according toanother embodiment.

FIG. 4B is a schematic cross-sectional view of an optical systemincluding the interchangeable lens of FIG. 4A.

FIG. 5A is a schematic cross-sectional view of a two-piece birefringentdevice with two structures of homogeneous thickness according to anembodiment.

FIG. 5B is a schematic diagram showing refractive index gradients in thebirefringent device of FIG. 5A.

FIG. 6A is a schematic plan view of a birefringent device with aplurality of rings in an outer portion of an active area of abirefringent device according to another embodiment.

FIG. 6B is a schematic diagram illustrating wavefront aberrations of anoptical system including the birefringent device of FIG. 6A for a firstpolarization direction for discussing effects of the embodiments.

FIG. 6C is a schematic diagram illustrating wavefront aberrations of anoptical system including the birefringent device of FIG. 6A for a secondpolarization direction orthogonal to the first polarization directionfor discussing effects of the embodiments.

FIG. 7A is a schematic diagram illustrating the out-of-focus pointspread function of the lens unit of FIG. 6A without birefringent device.

FIG. 7B is a schematic diagram illustrating the out-of-focus pointspread function of the lens unit of FIG. 6A with birefringent device.

FIG. 8 is a schematic cross-sectional view of a controllablebirefringent device according to a further embodiment.

FIG. 9 is a simplified flowchart of a method of manufacturing a lensunit or an objective according to another embodiment.

DESCRIPTION OF THE EMBODIMENTS

FIGS. 1A to 1D refer to an optical system 110 of an optical imagingdevice, for example a camera for consumer, industrial, surveillance ormedical applications or an apparatus including a camera function, forexample, a cellular phone or a PDA (personal digital assistant), by wayof example. According to an embodiment, the optical system 110 is aninterchangeable lens for a professional camera, for example a prime lensor a zoom lens.

The optical system 110 may include a lens unit 112 that images an objectonto an image capture unit 120 arranged in the image plane of the lensunit 112. The lens unit 112 may include a plurality of focusing anddefocusing single lenses and optical surfaces, lenses for correctinglens aberrations of the focusing and defocusing lenses as well asoptical filters. A front diameter of the lens unit 112 may be a diametertypical for mobile phones cameras, e.g., less than 10 mm. According toan embodiment, a front diameter of the lens unit 112 may be greater than40 mm. The lens unit 112 has an out-of-focus PSF (point spread function)with an intensity distribution that includes at least one ring-shapedside peak at a radial distance to a center point of the out-of-focuspoint spread function. A side peak intensity value of the at least onering-shaped side peak exceeds a local minimum value between thering-shaped side peak and the center peak by at least 50%, e.g., atleast 100%.

The image capture unit 120 may include an electronic pixel sensorconverting light into electric signals. According to other embodiments,the image capture unit 120 includes a light sensitive film and a filmdrive controlling a transport of the light sensitive film out of andback into a sprocket.

The optical system 110 further includes a birefringent device 115arranged in an optical path of the optical system 110, e.g., in front ofthe first lens surface or between at least a first portion 112 a of thelens unit 112 and the image capture unit 120. A second portion 112 b ofthe lens unit 112 may be arranged between the birefringent device 115and the image capture unit 120. The birefringent device 115 is mountablein the optical path of the optical system 110 and may include fixing andadjustment means at or near the periphery. An effective area of thebirefringent device 115 may be in the aperture (pupil) of the opticalsystem 110, wherein a center point of the birefringent device 115 isarranged at an optical axis 119 of the optical system 110. For example,the birefringent device 115 may be arranged close to the pupil plane ofthe optical system 110 or close to or at an aperture stop of the lensunit 112. The birefringent device 115 may be spatially separated fromfurther single lenses of the optical system 110 of FIG. 1A or maydirectly adjoin a single lens. The effective area of the birefringentdevice 115 affects the light guided through the optical system 110 froman entrance at a side oriented to the imaged scene to the image captureunit 120. The birefringent device 115 may include a birefringent layercoated on a substrate, wherein the birefringent layer may be dividedinto several annular zones. The orientation of the extraordinary axis(e.g. fast axis in negative uniaxial crystals) of the birefringent layermay be homogeneous within each annular zone but may be different betweenthe annular zones.

A phase function of the birefringent device 115 selectively attenuatesthe ring-shaped side peak in the intensity distribution of theout-of-focus PSF of the lens unit 112 by at least 25%, e.g., more than50% such that a total out-of-focus PSF of the optical system 110including the lens unit 112 and the birefringent device 115 has no orless pronounced ring-shaped side peaks compared to the out-of-focus PSFof the lens unit 112 alone. An out-of-focus light point is imaged into alight spot which brightness in substance gradually decreases withincreasing distance to the center point, hence without perceivable ringcomponent. At the same time, the birefringent device 115 effects only toa low, nearly non-perceivable degree the in-focus PSF and the MTF(modulation transfer function) of the optical system 100 such that thebirefringent device 115 facilitates a pleasant bokeh without additionalcorrection lenses and without or with only low adverse impact on thein-focus characteristics.

Typically, in professional cameras an apodization mask or additionalcorrection lenses shape the blur of out-of-focus image portions toobtain a more pleasant bokeh. Additional lenses result in heavier,longer and more expensive objectives. An apodization mask absorbs lightand may complicate autofocus mechanisms. Instead, the birefringentdevice 115 achieves a similar effect without loss of light, withoutadditional lenses, at fewer costs, at less weight as well as smallerdimensions and without adverse impact on autofocus systems.

The birefringent device 115 achieves a suitable pupil function of theoptical system by affecting the two fundamental orthogonal polarizationdirections of non-polarized or polarized light differently. In otherwords, the birefringent device 115 realizes a first pupil functionassigned to a first polarization direction and a second different pupilfunction assigned to a second polarization direction orthogonal to thefirst polarization direction, wherein the added phase profile to thefirst pupil function is a complex conjugate of the added phase profileof the second pupil function.

According to an embodiment, the birefringent material, the thickness ofa birefringent layer, and/or other optical properties, for example therefractive indices for two orthogonal polarization states, may be variedas a function of a distance to the center point of the birefringentdevice 115 in order to achieve the two different pupil functions.

The birefringent device 115 and its phase function may be rotationallysymmetric with respect to the center point such that a rotation by anangle of 360°/n, n=1, 2, 3 . . . , does not change the respectivefunction. For example, both phase functions may be point-symmetric suchthat a rotation by an angle of 360°/(2n), n=1, 2, 3 . . . , does notchange the respective function. According to an embodiment, both phasefunctions may be circularly symmetric and are functions of the distancefrom the center point only.

FIGS. 1B and 1C refer to a birefringent device 115 with circularlysymmetric structure, whereby the birefringent device 115 achievesdifferent pupil functions for orthogonal polarization directions byvarying the orientation of the fast axis of a birefringent materialacross the pupil plane. In a circularly symmetric birefringent device115 an orientation of an ordinary axis and an orientation of anextraordinary axis of the birefringent material may change radially withrespect to a center point on the optical axis of the optical system.

The birefringent device 115 may include a transparent, disc-shapedsubstrate 115 s and a birefringent layer or coating 115 c formed on atleast one of two parallel principal surfaces of the substrate 115 s. Thecoating 115 c may include an inner circular zone 210 and a plurality ofannular zones surrounding the circular zones 210. The number of circularzones may be greater than 10 or greater than 20, wherein the annularzones have different widths in a range from 50 μm to 2000 μm. Forsimplification, FIG. 1B shows only a first annular zone 211 directlyadjoining the circular zone 210, a second annular zone 212 directlyadjoining the first annular zone 211, a third annular zone 213 directlyadjoining the second annular zone 212 and a fourth annular zone 214directly adjoining the third annular zone 213.

The circular zone 210 and the annular zones 211-214 may include orconsist of the same birefringent material. According to an embodiment,the circular zone 210 is formed from the same birefringent material asthe annular zones 211-214, wherein in the circular zone 210, and theeven numbered annular zones, e.g., the second annular zone 212 and thefourth annular zone 214, corresponding birefringent axes of thebirefringent material are oriented parallel to each other. In the oddnumbered annular zones, e.g., in the first and third annular zones 211,213, the birefringent axes are rotated with respect to the correspondingbirefringent axes in the circular zone 210. According to an embodiment,in the odd numbered annular zones, e.g., in the first and third annularzones 211, 213, the birefringent axes may be rotated by 90 degrees withrespect to the corresponding birefringent axes in the circular zone 210such that the orientations of the birefringent axes are interchangedwith respect to the circular zone 210, the second annular zone 212 andthe fourth annular zone 214.

According to another embodiment, the extraordinary axis (e.g. fast axis)orientation in the circular zone 210 may be tilted to the fast axisorientation in both the even and the odd annular zones. For example, thefast axis orientation in odd annular zones 211, 213 may be tilted by 90degree and the fast axis orientation in the circular zone 210 may betilted by 45 degree with respect to the fast axis orientation in theeven annular zones 212, 214. Transitions between the annular zones andbetween the circular zone 210 and the first annular zone 211 may begradual or stepped.

According to a further embodiment the birefringent coating 115 cincludes an opening corresponding to the circular zone 210, wherein theopening may be filled with transparent material, such that the circularzone 210 of the birefringent device 115 has no or only low effect on thePSF. Since the ring-shaped side peak is mainly caused by the wavefrontshape in the outer region, e.g., the outer half of a lens system, thecircular zone 210 also does not contribute to the attenuation orsuppression of the ring-shaped side peak in the defocus PSF. When thelens is stopped down, e.g., the F-number is doubled, a birefringentdevice with a transparent central portion has no adverse impact on thein-focus MTF when a side-peak suppression in the PSF is not needed.

The birefringent device 115 may be a non-lensing device which doesneither converge nor diverge light beams passing through thebirefringent device 115. According to other embodiments, thebirefringent device 115 may additionally correct aberrations causedelsewhere in the optical system 100, for example in the lens unit 112.

FIG. 1D shows the out-of-focus PSF 294 of the optical system 110 withoutbirefringent device 115. The lens unit 112 is designed for a certaintarget in-focus PSF and for correcting chromatic and other aberrationsat costs of an out-of-focus PSF with an intensity distribution includingpronounced ring-shaped side peaks at a distance to the center point 201,wherein the peaks induce ring-shaped blur in the out-of-focus lightpoints. The birefringent device 115 of FIGS. 1B and 1C can be designedsuch that in the resulting out-of-focus PSF 292 of the optical system110 combining the lens unit 112 and the birefringent device 115,ring-shaped side peaks at a radius of approximately 130 μm, 155 μm and170 μm are reduced by at least 25%, respectively. The remainingring-shaped side peaks are significantly less pronounced.

At the same time, the birefringent device 115 hardly affects thein-focus PSF and keeps the MTF of the optical system 110 high. Accordingto an embodiment both the total in-focus PSF of the optical system 110with the birefringent device 115 and the in-focus PSF of the opticalsystem without birefringent device 115 approximate delta functions orGaussian curves with a variance of 0.1 μm²<σ²<0.35 μm²

FIG. 1E illustrates that the in-focus PSF 296 of the optical system 110with birefringent device 115 approximates to a high degree the in-focusPSF 295 of the optical system 110 without birefringent device 115. As aconsequence, the impact of the birefringent device 115 on the in-focuscharacteristics of the optical system 110 is low.

FIG. 1F plots the in-focus MTF 298 of the optical system 110 includingthe birefringent device 115 as well as the in-focus MTF 297 of theoptical system 110 without the birefringent device 115, wherein themodulation transfer is plotted against the spatial frequency in lp/mm,respectively. With the birefringent device 115 the MTF 298 drops byabout 0.2 at low spatial frequencies below approximately 20 lp/mm,whereas at high spatial frequencies exceeding 60 lp/mm the birefringentdevice 115 has less impact on the MTF of the optical system 110.

According to other embodiments, further annular zones with thebirefringent axes rotated by any angle between 0 degree and 90 degreewith respect to the corresponding axes in the circular zone 210 may beformed between rings of orthogonal orientation of the birefringent axes.

FIGS. 2A and 2B illustrate the way a birefringent device 115 achievesthe desired effect on the out-of-focus PSF. FIG. 2A shows a birefringentdevice 115 with a first annular zone 211 in which the birefringent axesare rotated by 90 degrees with respect to the corresponding axes in aninner circular zone 210 and a second, outer annular zone 212. Arrow 221represents the orientation which is parallel to the extraordinary axisn_(e) of a first annular zone 211 in the birefringent device 115 andarrow 222 the orthogonal orientation which is parallel to theextraordinary axis n_(e) of the annular zone 212 in the birefringentdevice 115.

As illustrated in the cross-sectional views of FIG. 2B the birefringentdevice 115 may include a transparent substrate 115 s, e.g., a glassdisk, and a coating 115 c on at least one of two opposite principalsurfaces of the substrate 115 s. The coated surface of the substrate 115s may be planar.

The upper portion of FIG. 2B shows a wavefront of light, which islinearly polarized along axis 221 of FIG. 2A and the lower portion ofFIG. 2B shows a wavefront of light, which is linearly polarized alongaxis 222. A phase difference Δφ in rad between wavefront componentspassing through the circular zone 210 and the annular zones 211, 212 isequal to 2*π*d*(n_(e)−n_(o))/λ, with d representing the thickness of thebirefringent coating 115 c and λ representing the wavelength of thelight. The birefringent device 115 manipulates the phase distributionsfor the ordinary and extraordinary beams such that the phase function ofthe birefringent device 115 attenuates ring-shaped side peaks in anintensity distribution of an out-of-focus PSF of a lens unit of anoptical system. The added phase distribution for one state ofpolarization, e.g., the ordinary beam, is the complex conjugate of theadded phase distribution for the other state of polarization, e.g., theextraordinary beam.

FIGS. 3A to 3C refer to arrangements of birefringent devices 115 withinoptical systems 110. In FIG. 3A, the birefringent device 115 is arrangedbetween an entrance of the optical system 100 and the complete lens unit112 focusing the incoming light that passes through the entrance on theimage capture unit 120. An effective area of the birefringent device 115is within the aperture of the optical system 100.

In FIG. 3B, the birefringent device 115 includes a plurality ofspatially separated sub-devices realizing different pupil sub-functions,wherein the sum of the different sub-functions gives the first andsecond pupil functions. For example, a birefringent device 115 may be atwo-piece device with a first sub device 115 a realizing a first phasefunction and a second sub device 115 b realizing a second phasefunction. The sub devices 115 a, 115 b may be arranged between theentrance of the optical device 100 and the image plane. According to theillustrated embodiment both sub devices 115 a, 115 b are arrangedbetween the lens unit 112 and the image capture unit 120.

The birefringent device 115 may include one or more transparentsubstrates and one or more birefringent layers or coatings on thetransparent substrates. The substrates may be or may include anytransparent element arranged in the optical path of the optical system100, for example one single lens, more than one single lens or one ormore transparent disks with planar surface and without light convergingor diverging properties. The target out-of-focus PSF may be realized bylocally varying a polarization dependent refractive index, for exampleby varying the orientation of the extraordinary axis of a birefringentmaterial or other physical and/or chemical properties of the coatingacross the effective area. In accordance with another embodiment, thecoating may have a thickness gradient in conformity with the respectivephase function of the birefringent device 115. Another embodiment maycombine variation of thickness and refractive index variation to realizethe intended out-of-focus PSF.

According to an embodiment, the coating may be a liquid crystal polymeraligned and fixed during manufacturing. For example, an alignment layermay align the liquid crystal polymer, wherein the orientation pattern ofthe alignment layer is fixed during a lithographic process, byirradiating the alignment layer under a photomask with linearlypolarized UV radiation. The liquid crystal polymer finally is cured byirradiating it with UV radiation.

FIG. 3C refers to a birefringent device 115 including a liquid crystallayer 155 sandwiched between glass substrates. The orientation of thefast axis in the liquid crystal layer 155 may be controllable by acontrol unit 180 that switches the orientation of the liquid crystalmolecules between two different directions to control the out-of-focusPSF and the behavior of blurred image portions.

FIG. 4A shows an embodiment of a camera 100 with an interchangeable lens300 including a portion of the optical system 110 of FIG. 1A. The camera100 further includes an image capture unit 120 arranged in a housing400.

FIG. 4B shows an embodiment of the interchangeable lens 300 incombination with the image capture unit 120 of the camera 100 of FIG.4A. The interchangeable lens 300 may include a first lens portion 112 awith three single lenses 121, 122, 123 arranged between the entranceopening of the interchangeable lens 300 and the birefringent device 115.A second lens portion 112 b with five further single lenses 125, 126,127, 128 and 129 is arranged between the birefringent device 115 and anexit opening of the interchangeable lens 300. The birefringent device115 may be arranged close to an aperture stop of the interchangeablelens 300. The image capture unit 120 may include an electronic pixelsensor and a color filter matrix, which may be arranged in front of orwhich may be directly deposited on the image capture unit 120. Aprotection glass 130 may cover the image capture unit 120.

Other embodiments may use any combination of crystal axes or fast axisdistortion, thickness variation and variation of optical properties. Allthese and further degrees of design freedom can be utilized in anoptimization procedure that aims at achieving the specific opticalproperties of a pleasant bokeh. The optimization may be carried out by asimulation model and some optimization algorithms, for example particleswarm optimization or damped least squares. The simulation model runs ona processor or a computer, wherein from the desired properties of theoptical imaging device an objective function is derived, that definesthe target of the optimization process.

For example, FIGS. 5A and 5B refer to a birefringent device 115including a first structure 115 x in which the refractive indexeffective for the first polarization state changes with increasingdistance to a center point and with a second structure 115 y in whichthe refractive index effective for the second polarization state changeswith increasing distance to the center point. In accordance with theillustrated embodiment, the refractive indices n1(r), n2(r) may changein discrete or shallow steps between the center point and the radius Rof the effective area of the birefringent device 115 in a circularlysymmetric manner.

FIGS. 6A to 6C refer to a circularly symmetric birefringent device 115which effective area is a circle with a radius R. The birefringentdevice 115 includes an inner circular portion 210 and a plurality ofannular zones 219, wherein the radii of all annular zones 219 aregreater than R/2. The birefringent material is aligned homogeneouslywithin each annular zone 219.

According to an embodiment, the angle between the alignment directionsof adjacent annular zones 219 may be any value from 0 degree to 180degree. According to an embodiment, the angle between the alignmentdirections of adjacent annular zones 219 is 90 degree. The alignmentdirection of a uniaxial birefringent material is equivalent to thedirection of the extraordinary axis of that material. Alteration of theordinary and extraordinary axes in the outer annular zones 219 hasmainly impact on an outer portion of the out-of-focus PSF, wheretypically the out-of-focus PSFs of lens units show an intensitydistribution with ring-shaped side peaks.

FIG. 6B shows the wavefront for the horizontal polarization directionand FIG. 6C shows the wavefront for the vertical polarization directionfor the birefringent device 115 in case of an angle of 90 degreesbetween the extraordinary axes of adjacent annular zones.

FIG. 7A shows the out-of-focus PSF of a lens unit without birefringentdevice 115. Sharp dark-to-bright steps may point to a ring-shapedout-of-focus blur.

FIG. 7B shows the total out-of-focus PSF of the same lens unit incombination with a suitable circular symmetric birefringent deviceadapted to the lens unit. The birefringent device attenuates thering-shaped side peak in the intensity distribution.

FIG. 8 shows a birefringent device 115 including a liquid crystal layer155 as birefringent element. The birefringent device 115 may furtherinclude two transparent carrier substrates 152, 158 arranged inparallel, wherein the liquid crystal layer 155 fills a gap between thetwo carrier substrates 152, 158. One or both of the carrier substrates152, 158 may include an alignment layer 154, 156 that aligns the liquidcrystal molecules in the liquid crystal layer 155 in a predefinedalignment direction. The alignment direction may be position-dependent,for example may vary circularly symmetric to a centre point. Thepredefined alignment directions may define the phase function of thebirefringent device 115 completely or may at least contribute to thedefinition of the phase function.

According to an embodiment, the alignment layers are based on polymersand the orientation pattern of at least one of the alignment layers 154,156 may be fixed by linear photopolymerization. The liquid crystal layer155 contains a liquid crystal and the orientation pattern of the liquidcrystal is defined by the alignment layers. The birefringent device 115may further include one or more transparent electrodes 159 arranged atleast on one of the carrier substrates 152, 158.

A control unit 180 may be connected with the transparent electrodes 159and may control the orientation of the crystal axes of the liquidcrystal layer 155 by applying suitable voltages to the transparentelectrodes 159 in response to a user action or a change of an internalstate. For example, the user may turn on/off a switch at a camerahousing or an internal processor for automated control may output acontrol signal in order to trigger the control unit 180 to change thephase function of the liquid crystal layer 155 from a first state to asecond state and vice versa, wherein in the first state the liquidcrystal attenuates the ring-shaped side peak in the out-of-focus pointspread function of the lens unit by at least 25 percent and in thesecond state an effect of the liquid crystal on the out-of-focus pointspread function is reduced with respect to the first state to degreeperceivable by the user.

Separately controllable sections 159 a, 159 b, 159 c of at least one ofthe transparent electrodes 159 may be assigned to different regions ofthe liquid crystal layer 155, for example to a circular region and oneor more annular regions surrounding the circular region.

In accordance with an embodiment, the control unit 180 is configured tocontrol the orientation of the crystal axes continuously such that theorientation of the complete liquid crystal layer 155 or a section of theliquid crystal layer 155 can be adjusted to an arbitrary angle between aminimum and a maximum value. According to another embodiment the controlunit 180 controls the orientation in steps. For example, the liquidcrystal layer 155 is a ferro-electric liquid crystal and the controlunit 180 switches the orientation of the crystal axes of the liquidcrystal layer 155 between a first orientation and a second orientationdiffering from the first orientation by, e.g., 90 degree.

For example, the control unit 180 may switch the orientation of the fastaxis of the liquid crystal layer 155 out of a plane parallel to surfacesof the carrier substrates 152, 158 and/or parallel to the plane parallelto the surfaces of the carrier substrates 152, 158. For example, theliquid crystal layer 155 is a nematic liquid crystal in which theorientation of the fast axis in the liquid crystal layer 155 follows theelectric field vector. As the electric field vector is perpendicular tothe surfaces of the carrier substrates 152, 158, the crystal axis of theliquid crystal layer 155 will tilt out of the plane parallel to thesurfaces of the carrier substrates 152, 158. As a result the effectiverefractive index for light passing the liquid crystal layer 155 will bedecreased in case n_(e) is greater n_(o) or increased in case n_(o) isgreater n_(e). A phase difference between the two linear Eigenpolarization states will change accordingly. As a result, the appliedvoltage controls the phase function of the birefringent device 115.

The transparent electrodes may be divided into segments, wherein eachsegment can be driven by a separate voltage such that different phasepatterns can be applied at a birefringent device 115. According to anembodiment, the voltage pattern can be adjusted for different modes. Forexample, one mode may show smooth defocus blur at costs of lowerin-focus MTF and a second mode may exhibit high in-focus MTF but lesssmooth blur. A further mode may correspond to an off-state with almostno residual birefringence for perpendicular incident light beams suchthat the birefringent device 115 has no perceivable impact on the shapeof both the in-focus PSF and the out-of-focus PSF.

FIG. 9 refers to a method of manufacturing an optical system. Anout-of-focus point spread function of a lens unit with a plurality oflenses is determined, e.g., measured or derived from a virtual model ofthe lens unit (902). By using a numerical optimization method, apolarization dependent phase alteration for the pupil function isobtained that, when combined with the lens unit, attenuates thering-shaped side peak in an intensity distribution of the out-of-focuspoint spread function by at least 25 percent. For minimizing thering-shaped side peaks, the numerical optimization method varies twopupil functions with a complex conjugate portion, with minimum effect onan in-focus point spread function of the lens unit (904). A birefringentdevice may be manufactured that embodies a phase function obtained fromthe complex conjugate phase portion of the two pupil functions. Thebirefringent device may be mounted into the optical path of the opticalsystem.

Note that the present technology can also be configured as describedbelow.

(1) An optical system including:

-   -   a lens unit including a plurality of lenses, wherein an        out-of-focus point spread function of the lens unit includes an        intensity distribution with a ring-shaped side peak at a radial        distance to a center point; and    -   a birefringent device in an optical path of the optical system,        wherein the birefringent device is adapted to selectively        attenuate the ring-shaped side peak in the out-of-focus point        spread function of the lens unit.

(2) The optical system according to (1), wherein

-   -   the pupil function of the lens unit is rotational symmetric with        respect to the optical axis.

(3) The optical system according to (1) or (2), wherein

-   -   the pupil function of the lens unit is circularly symmetric with        respect to the optical axis.

(4) The optical system according to (1) to (3), wherein

-   -   the pupil function of the optical system comprises a first pupil        function assigned to a first polarization state and a second        pupil function assigned to a second polarization state        orthogonal to the first polarization state, and the first pupil        function includes a phase portion which a is a complex conjugate        of a phase portion included in the second pupil function.

(5) The optical system according to (1) to (4), wherein

-   -   in the birefringent device an orientation of an ordinary axis        and an orientation of an extraordinary axis change radially with        respect to the optical axis of the optical system, respectively.

(6) The optical system according to (1) to (5), wherein

-   -   the birefringent device comprises a circular inner zone and at        least two annular zones surrounding the circular inner zone,        respectively, wherein the circular inner zone and the        neighboring annular zone as well as neighboring annular zones        have different orientations of the ordinary and extraordinary        axes of the birefringent material.

(7) The optical system according to (6), wherein

-   -   a radius of the circular inner zone is at least half a radius of        an effective area of the birefringent device.

(8) The optical system according to (6) or (7), wherein

-   -   the birefringent device comprises at least ten annular zones.

(9) The optical system according to (6) to (8), wherein

-   -   the orientation of an ordinary axis in an annular zone        corresponds to the orientation of an extraordinary axis in a        neighboring circular inner zone or annular zone.

(10) The optical system according to (1) to (9), wherein

-   -   the birefringent device comprises a liquid crystal layer.

(11) The optical system according to (10) further including

-   -   a control unit configured to control the liquid crystal layer in        response to a user input or an internal state, wherein in a        first state a phase function of the liquid crystal layer is        configured to attenuate the ring-shaped side peak in the        intensity distribution of the out-of-focus point spread function        of the lens unit and in a second state an effect of the liquid        crystal layer on the out-of-focus point spread function is        reduced with respect to the first state.

(12) The optical system according to (1) to (11), further including

-   -   the birefringent device comprises a first structure in which a        refractive index effective for the first polarization state        changes with increasing distance to the optical axis and a        second structure in which a refractive index effective for the        second polarization state changes with increasing distance to        the optical axis.

(13) The optical system according to (1) to (12), wherein

-   -   the birefringent device comprises a birefringent coating on an        element of the lens unit.

(14) The optical system according to (1) to (13), wherein

-   -   the birefringent device is adapted to attenuate the ring-shaped        side peak in the out-of-focus point spread function of the lens        unit by at least 25 percent.

(15) An interchangeable lens including:

-   -   a lens unit including a plurality of lenses, wherein an        out-of-focus point spread function of the lens unit includes an        intensity distribution with a ring-shaped side peak at a radial        distance to a center point; and    -   a birefringent device in an optical path of the interchangeable        lens, wherein the birefringent device is adapted to selectively        attenuate the ring-shaped side peak in the out-of-focus point        spread function of the lens unit.

(16) The interchangeable lens according to (15), wherein

-   -   the birefringent device is adapted to attenuate the ring-shaped        side peak in the out-of-focus point spread function of the lens        unit by at least 25 percent.

(17) A camera, including:

-   -   a lens unit including a plurality of lenses, wherein an        out-of-focus point spread function of the lens unit includes an        intensity distribution with a ring-shaped side peak at a radial        distance to a center point; and    -   a birefringent device in an optical path of the lens unit,        wherein the birefringent device is adapted to selectively        attenuate the ring-shaped side peak in the out-of-focus point        spread function of the lens unit.

(18) The camera according to (17), wherein

-   -   the birefringent device is adapted to attenuate the ring-shaped        side peak in the out-of-focus point spread function of the lens        unit by at least 25 percent.

(19) A method of manufacturing an optical system, the method including:

-   -   determining an out-of-focus point spread function of a lens unit        that comprises a plurality of lenses, and    -   determining, by using a numerical optimization method, a        polarization dependent phase alteration for the pupil function,        the phase alteration attenuating a ring-shaped side peak in an        intensity distribution of the out-of-focus point spread function        of the lens unit by at least 25 percent when combined with the        phase portion of the pupil function of the lens unit, the        numerical optimization method varying two complex conjugate        phase alterations in combination with the pupil function of the        lens unit for minimizing the ring-shaped side peaks at minimum        effect on an in-focus point spread function of the lens unit.

(20) The method according to (19), further including

-   -   manufacturing a birefringent device embodying the two complex        conjugate phase alterations.

(21) The method according to (20), further including

-   -   mounting the birefringent device in the optical path of the        optical system.

Thus, the foregoing discussion discloses and describes merely exemplaryembodiments of the present disclosure. As will be understood by thoseskilled in the art, the present disclosure may be embodied in otherspecific forms without departing from the spirit or essentialcharacteristics thereof. Accordingly, the present disclosure is intendedto be illustrative, but not limiting of the scope of the disclosure, aswell as other claims. The disclosure, including any readily discerniblevariants of the teachings herein, defines, in part, the scope of theforegoing claim terminology.

The present application claims priority to European Patent Application15000940.5 filed by the European Patent Office on 31 Mar. 2015, theentire contents of which being incorporated herein by reference.

The invention claimed is:
 1. An optical system comprising: a lens unitcomprising a plurality of lenses, wherein an out-of-focus point spreadfunction of the lens unit includes an intensity distribution with aring-shaped side peak at a radial distance to a center point; and abirefringent device in an optical path of the optical system, whereinthe birefringent device is adapted to selectively attenuate thering-shaped side peak in the out-of-focus point spread function of thelens unit.
 2. The optical system according to claim 1, wherein the pupilfunction of the lens unit is rotational symmetric with respect to theoptical axis.
 3. The optical system according to claim 1, wherein thepupil function of the lens unit is circularly symmetric with respect tothe optical axis.
 4. The optical system according to claim 1, whereinthe pupil function of the optical system comprises a first pupilfunction assigned to a first polarization state and a second pupilfunction assigned to a second polarization state orthogonal to the firstpolarization state, and the phase portion of the first pupil function isa complex conjugate of the phase portion of the second pupil function.5. The optical system according to claim 1, wherein in the birefringentdevice an orientation of an ordinary axis and an orientation of anextraordinary axis change radially with respect to the optical axis ofthe optical system, respectively.
 6. The optical system according toclaim 1, wherein the birefringent device comprises a circular inner zoneand at least two annular zones surrounding the circular inner zone,respectively, wherein the circular inner zone and the neighboringannular zone as well as neighboring annular zones have differentorientations of the ordinary and extraordinary axes of the birefringentmaterial.
 7. The optical system according to claim 6, wherein a radiusof the circular inner zone is at least half a radius of an effectivearea of the birefringent device.
 8. The optical system according toclaim 6, wherein the birefringent device comprises at least ten annularzones.
 9. The optical system according to claim 6, wherein theorientation of an ordinary axis in an annular zone corresponds to theorientation of an extraordinary axis in a neighboring circular innerzone or annular zone.
 10. The optical system according to claim 1,wherein the birefringent device comprises a liquid crystal layer. 11.The optical system according to claim 10, further comprising a controlunit configured to control the liquid crystal layer in response to auser input or an internal state, wherein in a first state a phasefunction of the liquid crystal layer is configured to attenuate thering-shaped side peak in the intensity distribution of the out-of-focuspoint spread function of the lens unit and in a second state an effectof the liquid crystal layer on the out-of-focus point spread function isreduced with respect to the first state.
 12. The optical systemaccording to claim 1, further comprising the birefringent devicecomprises a first structure in which a refractive index effective forthe first polarization state changes with increasing distance to theoptical axis and a second structure in which a refractive indexeffective for the second polarization state changes with increasingdistance to the optical axis.
 13. The optical system according to claim1, wherein the birefringent device comprises a birefringent coating onan element of the lens unit.
 14. The optical system according to claim1, wherein the birefringent device is adapted to attenuate thering-shaped side peak in the out-of-focus point spread function of thelens unit by at least 25 percent.
 15. An interchangeable lenscomprising: a lens unit comprising a plurality of lenses, wherein anout-of-focus point spread function of the lens unit includes anintensity distribution with a ring-shaped side peak at a radial distanceto a center point; and a birefringent device in an optical path of theinterchangeable lens, wherein the birefringent device is adapted toselectively attenuate the ring-shaped side peak in the out-of-focuspoint spread function of the lens unit.
 16. The interchangeable lensaccording to claim 15, wherein the birefringent device is adapted toattenuate the ring-shaped side peak in the out-of-focus point spreadfunction of the lens unit by at least 25 percent.
 17. A camera,comprising: a lens unit comprising a plurality of lenses, wherein anout-of-focus point spread function of the lens unit includes anintensity distribution with a ring-shaped side peak at a radial distanceto a center point; and a birefringent device in an optical path of thelens unit, wherein the birefringent device is adapted to selectivelyattenuate the ring-shaped side peak in the out-of-focus point spreadfunction of the lens unit.
 18. The camera according to claim 17, whereinthe birefringent device is adapted to attenuate the ring-shaped sidepeak in the out-of-focus point spread function of the lens unit by atleast 25 percent.